Integrated circuits (IC) are typically mounted onto printed circuit (PC) boards for use. In one implementation, called direct chip attach, the IC is attached directly on the PC board. Another common form mounts the IC on a substrate. Subsequently, a lid, encapsulant or heat sink may be applied over the IC/substrate combination. The IC and substrate assembly are typically called an IC package. For simplicity, the IC package will be discussed without reference to the lid, encapsulant or heat sink. The IC package is then attached onto a PC board for use. IC's and IC packages are typically attached to the PC board using solder. The solder makes the electrical connection between the IC/IC package and the PC board as well as the mechanical connection between the IC/IC package and the PC board. For the direct chip attach method the solder is typically referred to as solder bumps and for ball grid array packages the solder is typically in the form of solder balls. During the soldering process the IC/IC package, the PC board, and the solder are heated until the solder melts. When the solder cools below its freezing point the solder forms a bond between the IC/IC package and the PC board. Once the bond is complete the assembly is cooled to room or operating temperature.
Because the IC and some of the IC packages and the PC board are made of different materials that have different coefficients of thermal expansion, the change in length for a given change in temperature is different between the IC/IC package and the PC board. The substrate in an IC package may have a different coefficient of thermal expansion than the PC board. The PC board typically has a higher coefficient of thermal expansion than the IC, causing a greater length change for a given temperature change. For example, when the solder (106) freezes at a temperature of around 183 degrees C., the joint between the IC (102) and the PC board (104) is stress free (see FIG. 1a). For clarity, FIG. 1a only shows two solder bumps attaching the IC to the PC board, however, in practice there will be a plurality of solder bumps. As the assembly continues to cool, the IC changes in length at a slower rate than the PC board does. This difference in the shrink rate between the IC and the PC board induces stress into the solder bumps joining the assembly as well as induces stress into both the PC board and the IC. The stress in the assembly can cause warping of the PC board (see FIG. 2), shearing of the solder bumps, crack propagation, delamination of the PC board, and other undesirable effects. These problems typically occur in both the direct chip attach IC (see FIG. 1a) and the IC package attached to a PC board (see FIG. 1b). FIG. 2 shows an exaggeration of the distortion in the IC and the PC board for illustration purposes only; the actual distortion would not typically be visible to the unaided eye. Even when the stress does not cause an immediate failure it may reduce the life of the assembly through fatigue. Fatigue is caused by differential expansion of the solder joint due to the change in temperature of the IC or IC package, the PC board and the environment during normal operation.
One partial solution to these problems is to fill the gaps between the IC and the PC board or the gap between the IC and substrate with an epoxy or other non-conductive material. The addition of epoxy or other materials is typically called underfill. Underfill merely provides an additional load-carrying component. By carrying part of the load, the underfill (302) reduces the load in the solder balls (304) and spreads the stress in the IC (306) and PC board or substrate (308) across the surface of the interface (see FIG. 3). Unfortunately the underfill is susceptible to delamination from the IC, substrate and/or the PC board surfaces. Applying underfill must be done at elevated temperatures and causes an additional step in the manufacturing process. Underfill can be used in the joint between an IC and PC board, in the direct chip attach method, as well as the joint between the substrate and the IC in an IC package assembly.
Accordingly, there is a need for reduced stress in the joint between an IC and a PC board. There is a need for an assembly that reduces stress in the joint between an IC and a substrate. Finally there is a need for an assembly that reduces stress in the joint between a substrate and a PC board